1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus using the fixing device. More particularly, the present invention relates to a fixing device including a heat roller supported by bearings.
2. Discussion of the Background
In image forming apparatuses, such as copying machines, printers, and the like, there is known an image forming apparatus provided with a fixing device that conveys a transfer sheet while nipping the transfer sheet with a heat roller and a pressure roller to affix a toner image on the transfer sheet, which is transferred onto the transfer sheet from a photoconductor drum. Namely, heat and pressure are applied to the transfer sheet in the sheet conveying process by the fixing device to affix the toner image on the transfer sheet.
A known fixing device illustrated in FIG. 11 is an example of such a fixing device. In the fixing device illustrated in FIG. 11, an elastic piece 103 having claw-shaped hooks 102 on respective upper ends is formed in a body of a C-shaped bearing 101 by which each end of a heat roller 100 is rotatably supported. The bearing 101 is secured to a supporting member 104 such as a housing by the hooks 102 elastically hooked to the supporting member 104.
When the fixing device is constructed, at first the bearing 101 is secured to the supporting member 104, and the heat roller 100 is fit into the bearing 101, by elastically expanding a distance between both ends of the bearing 101. Then, the pressure roller 105 is brought into contact with the heat roller 100.
The operation fitting the heat roller 100 into the bearing 101 is performed by pressing the heat roller 100 in a direction orthogonal to the axis of rotation of the heat roller 100. Thus, the fixing device is intended to be easily and automatically constructed.
In a fixing device having a heat roller and a pressure roller, a heater is generally provided inside the heat roller, and the heat roller and the pressure roller are set in a housing. The heater is composed of a quartz glass tube, both ends of which are sealed, and a heat wire provided in the quartz glass tube. The heater is further composed of a lead wire, an end of which is connected to the heat wire through a metal foil and the other end of which extends from the quartz glass tube, and the like parts. A heater terminal is connected to the other end of the lead wire that extends from the quartz glass tube and is affixed to the housing by a screw. A wire harness, which supplies electric power to the heater, is connected to the heater terminal.
Since the fixing device emits a large amount of heat, the housing is required to have thermostability and rigidity and is made of glass-containing synthetic resin. It is a current trend that the bearing is also made of a synthetic resin for the purposes of cost-reduction and weight-reduction. However, in fact the bearing is generally made of expensive synthetic resin, such as PPS (polyphenylene sulfide).
In addition, the heat roller is charged with static electricity generated by friction between the heat roller and the transfer sheet. Therefore, the heat roller is made of electric conductive material, such as metal, or the like. In order to ground the static electricity stored in the heat roller through the bearing, the bearing is also required to be electroconductive. Accordingly, the material used in the bearing becomes more and more expensive.
In FIG. 11, when the bearing 101 is attached to or detached from the supporting member 104, the elastic piece 103 has to be bent to be hooked to the supporting member 104. Therefore, the length of the elastic piece 103 must be relatively long so that the elastic piece 103 becomes flexible. An amount of the material necessary to form a bearing 101 thereby increases resulting in increase manufacture costs.
Further, the heater generates a relatively large amount of heat when image forming operations are performed. Therefore the housing is expanded by the heat, and the heater terminals affixed to the housing tend to move in such a direction to that the heater terminals pull the lead wires.
Therefore, various problems occur. For example, the heater terminal can become disconnected from the lead wire at the connection part thereof, the metal foil provided in the quartz glass tube can become disconnected from the lead wire at the connection part thereof, and the quartz glass tube can become cracked, thereby damaging the heater.
Accordingly, an object of the present invention is to provide a fixing device including a bearing to support a heat roller, which has a simple structure so as to be easily assembled, and which is made at a low cost. Another object of the present invention is to provide an image forming apparatus using the fixing device.
Briefly, these objects, and other objects of the present invention as hereinafter will become more readily apparent, can be attained by a novel fixing device that includes a bearing including a C-shaped part; and a projection projected upwardly from a central part of an outer circumferential surface of the C-shaped part, in which the bearing is made of a synthetic resin having elasticity and conductivity, and a distance between both ends of the C-shaped part is elastically expanded. The fixing device further including a heat roller configured to be rotatably supported by the bearing at an upper part of an outer surface of each end of the heat roller in which the outer surface of each end of the heat roller which contacts the bearing, is made of an electroconductive material; a pressure roller configured to be located downward relative to the heat roller and that is driven by being in pressure contact with the heat roller; a housing configured to cover peripheries of the heat roller and the pressure roller and having an inserting part into which the projection of the bearing is detachably inserted from a pressure roller side; and a ground plate configured to attach to the housing and including an elastic piece that detachably hooks the C-shaped part of the bearing, and a ground piece that is grounded.
The housing generally includes an inserting part into which the projection of the bearing is detachably inserted from a pressure roller side, and a hooking pawl that detachably hooks the C-shaped part.
An additional embodiment of the fixing device includes a heat roller including a heater therein; a lead wire configured to extend from each end of the heater; and a heater terminal configured to be provided at each end of the lead wire and which is fixed to the housing, wherein a flexible space in which the heater terminal is capable of bending along a longitudinal direction of the heater is formed on at least one fixing position of the heater with the housing.
A further embodiment of the fixing device includes a heat roller that includes a heater; a thermal fuse configured to be connected to a circuit to supply electricity to the heater and is disposed adjacent to an outer circumference of the heat roller at an opposite side to the pressure roller while keeping an insulation distance between the heat roller and the thermal fuse based on the safety standard; and a thermostable stopper configured to keep an insulation distance between the heat roller and the thermal fuse within such a distance that an insulation strength based on the safety standards is kept even when the bearing melts.